Dual graphics on APU performs terribly

My laptop is a ThinkPad E575, with 7th gen A12 9700P processor and 8G DDR4 2133 on broad. I recently bought a new 8G DDR4 2400 memory bank and intended to set up a two channel memory set. Then I found there's a dual graphics sign when I play Overwatch(I didn't know DG before, I just wanted better performance), however, fps drops from 40 to less than 20. What's wrong and how can I fix it?